1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to the case of a lithium secondary battery which hermetically seals and surrounds a battery body generating a current.
2. Description of the Related Art
As electronic devices such as camcorders, cellular phones or notebook type computers have been made smaller in size and lighter in weight, there has been considerable demand for small and lightweight secondary batteries with high energy density which can be used for these electronic devices. As such, a lithium secondary battery has been attracting attention as a high energy density battery. According to the kind of electrolyte used, a lithium secondary battery can be classified into a lithium ion battery which uses a liquid electrolyte and a lithium ion polymer battery which uses a solid electrolyte.
The lithium ion polymer battery includes a battery body for generating current, which is made by stacking a cathode, an anode and a separator. An electrode terminal serving as an electrical path to the outside is connected to the battery body. Parts of the battery body and the electrode terminal are hermetically sealed in an insulating case. The case of the lithium ion polymer battery hermetically seals the battery body such that thermal adhesive material layers, an innermost layer of the battery body, are adhered to each other.
FIG. 1 is an exploded perspective view showing an example of a conventional lithium ion polymer battery. Referring to FIG. 1, the lithium ion polymer battery includes a battery body 11 having a cathode plate, an anode plate and a separator, and a case 12 surrounding the battery body 11, heretically sealing the body. Electrode tabs 14 and 14' serving as an electrode path for inducing the current generated in the battery body 11 to the outside are installed such that a predetermined length of them is exposed outside the case material 12.
The case 12 is made of an outer material such as an insulating film having a stacked structure, as shown in FIG. 2.
Referring to FIG. 2, the outer material is formed such that a thermal adhesive material layer 21, an innermost layer of a battery body 20, has an aluminum (Al) layer 22, a polyethylene layer 23, and a polyester layer 24 sequentially deposited thereon. Here, the thermal adhesive material layer 21 is formed of SURLYN (Dupont Inc.) obtained by adding Na, K, Mg or Zn to poly(ethylene-co-acrylic acid). The Al layer 22 on the thermal adhesive material layer 21 prevents permeation of moisture and loss of an electrolytic solution. The polyethylene layer 23 strengthens the functions of an outermost layer and improves adhesiveness. The polyester layer 24, the outermost layer, protects the battery against external shock.
The case 12 formed of the above-described outer material hermetically seals the battery such that the battery body 11 is inserted into the case 12, with only parts of the electrode taps 14 and 14' being exposed, and then the case is folded in half. Then, heat and pressure are applied to the case 12 to hermetically seal the battery such that thermally adhesive materials of the edges in the upper and lower portions of the case 12 are adhered to each other.
As described above, the case is bonded by thermally adhering parts of the innermost layer surrounding the battery body 20. However, since the outer material of the conventional lithium secondary battery is relatively thin and the Al layer 22 is particularly thin, that is, about 9 .mu.m, it is not easy to fabricate the outer material of the conventional lithium secondary battery. Also, the polyester layer 24, the outermost layer, is susceptible to defects during molding, such as cracking.
Further, in the outer material of the conventional lithium secondary battery, when the innermost layer is thermally adhered, the adhesion efficiency may be poor, depending on a slight change in processing conditions including pressure, temperature and time, which causes leakage of the electrolytic solution in the lithium secondary battery.